Cysteine thiolate (Cys-S−) is a highly reactive group in proteins serving structural, regulatory and enzymatic functions. A unique reaction that distinguishes thiolates from other protein functional groups is the reversible oxidation by reactive oxygen and nitrogen species (ROS/RNS). Reaction products include thiyl radicals, sulfenic acid (—SOH), disulfides (—S—S—), sulfinic acid (—SO2H) and sulfonic acid (—SO3H) species. The highly reactive thiyl radicals decay rapidly and form disulfides. The metastable—SOH species[1] also favors conversion into more stable species. Disulfides are formed through —SOH reaction with free thiol (—SH) biomolecules like glutathione and cysteines in protein. Additionally, —SOH was shown to react with amine (—NH2) from lysine residues or amide (—NHCO—) in the protein backbone to form sulfenamides (—S—N).[2] Irreversible oxidation to sulfinic and sulfonic states can occur at higher concentrations of ROS. Sulfiredoxin-catalyzed reduction of —SO2H to —SOH in peroxiredoxins is the only currently known exception to this irreversibility[3] On the contrary, —SOH, disulfide and sulfenamide products can be reversibly reduced to —SH by thiol-containing reductants (glutathione, DTT) and phosphines (TCEP).[4] This reversibility is akin to phosphorylation, a critical regulatory protein posttranslational modification. Like phosphorylation, oxidation regulates protein functions[5] and impacts the relay of signaling and metabolic events through redox-mediated processes.[6] The transient nature and reversibility of —SOH in proteins makes their detection challenging. However, given the importance of studying the oxidized proteome, there is an increased interest in developing analytical methods for detection of the —SOH proteins and the sites of cysteine oxidation.[7] Most commonly available methods for selective labeling of —SOH rely on the use of dimedone (5,5-dimethyl-1,3-cyclohexanedione) or dimedone-like derivatives, that have complex synthesis schemes.[8] Facile, two-step synthesis and kinetic characterization of new 1,3-cyclopentanedione-based chemical probes for selective labeling of —SOH in proteins were described recently by the applicants group.[9] The synthesis route relies on highly efficient Michael addition of thiol containing tags or linkers to 4-cyclopentene-1,3-dione, the unsaturated derivative of 1,3-cyclopentanedione.[9-10]